100358_Stringtown_Rd_-_GEOTECH_REPORT.pdf

DRAFT

REPORT OF GEOTECHNICAL ENGINEERING SERVICES

Stringtown Road Bridge Replacement Washington County, Oregon

For Washington County Department of Land Use and Transportation c/o David Evans and Associates December 4, 2023

Project: WashCo-97-01

December 4, 2023

Washington County Department of Land Use and Transportation c/o David Evans and Associates 2100 South River Parkway Portland, OR 97201

Attention: Cameron Grile, P.E.

Report of Geotechnical Engineering Services Stringtown Road Bridge Replacement

Washington County, Oregon Project: WashCo-97-01

NV5 is pleased to submit this report of geotechnical engineering services for the proposed Stringtown Road Bridge Replacement project in Washington County, Oregon. This report presents a description of our services and site conditions; a summary of field exploration and laboratory testing; and recommendations for design and construction of bridge foundations and pavement design parameters.

We appreciate the opportunity to be of service to Washington County. Please contact us if you have questions regarding this report.

Sincerely, NV5

Jeffery D. Tucker, P.E., G.E.

Principal Engineer cc: Amanda Owings, Otak, Inc.

JDT:kt

Attachments

One copy submitted

Document ID: WashCo-97-01-120423-geor.docx

© 2023 NV5. All rights reserved.

Kristen.Tebbe Draft i WashCo-97-01:120423

EXECUTIVE SUMMARY

Based on information provided by Otak, Inc. and the results of our exploration, laboratory testing, and analyses, it is our opinion that the project is feasible. The primary geotechnical considerations for the project are summarized as follows:

The key geotechnical consideration for the proposed culvert will be settlement of the highly compressible native clay soil from proposed loading from headwalls and culvert foundations.

We recommend that the native soft and medium stiff clay be removed to a depth of 2 feet below the culvert and headwall foundations and replaced with granular backfill or stone embankment material.

Due to the presence of soft and wet soil, flatter than normal temporary cuts may be required, resulting in larger backfill quantities. We recommend that a contingency be in place for larger than anticipated backfill quantities. As an alternative, temporary shoring can be used to reduce temporary excavation quantities.

The existing soil at the site is fine grained and easily disturbed by construction traffic. If not carefully executed, site preparation, grading, and culvert roadway excavation in this soil can create extensive soft areas and significant subgrade repair costs can result.

WashCo-97-01:120423

TABLE OF CONTENTS

ACRONYMS AND ABBREVIATIONS

1.0 INTRODUCTION 1

2.0 PURPOSE AND SCOPE 1

3.0 SITE CONDITIONS 1

3.1 Geologic Setting 1

3.2 Surface Conditions 2

3.3 Subsurface Conditions 2

4.0 CONCLUSIONS 4

5.0 DESIGN 4

5.1 Permanent Slopes 4

5.2 Seismic Considerations 4

5.3 Culvert Foundation Recommendations 6

5.4 Retaining Structures 6

5.5 Drainage 8

5.6 Pavement Design Recommendations 8

6.0 CONSTRUCTION 9

6.1 Erosion Control 9

6.2 Site Preparation 9

6.3 Wet Weather Considerations 10

6.4 Excavation 11

6.5 Materials 12

7.0 OBSERVATION OF CONSTRUCTION 15

8.0 LIMITATIONS 15

REFERENCES 17

FIGURES

Vicinity Map Figure 1 Site Plan Figure 2

APPENDIX

Field Explorations A-1 Laboratory Testing A-1 Exploration Key Table A-1 Soil Classification System Table A-2 Rock Classification System Table A-3 Boring Log Figure A-1 Atterberg Limits Test Results Figure A-2 Grain-Size Test Results Figure A-3 Summary of Laboratory Data Figure A-4 SPT Hammer Calibration

WashCo-97-01:120423

ACRONYMS AND ABBREVIATIONS

AADT annual average daily traffic AASHTO American Association of State Highway and Transportation Officials AC asphalt concrete ACP asphalt concrete pavement AOS apparent opening size ASTM American Society for Testing and Materials BGS below ground surface ESAL equivalent single-axle load g gravitational acceleration (32.2 feet/second2) H:V horizontal to vertical ODOT Oregon Department of Transportation OSHA Occupational Safety and Health Administration OSSC 2021 Oregon Standard Specifications for Construction pcf pounds per cubic foot PG performance grade PGA peak ground acceleration psf pounds per square foot psi pounds per square inch RQD rock quality designation SPT standard penetration test

1 WashCo-97-01:120423

1.0 INTRODUCTION

This report presents the results of NV5’s geotechnical engineering evaluation for the proposed Stringtown Road Bridge Replacement project in Washington County, Oregon. The site relative to surrounding features is shown on Figure 1. The project layout is shown on Figure 2. Acronyms and abbreviations used herein are defined above, immediately following the Table of Contents.

Based on information provided by Otak, Inc. and Washington County, the project will consist of replacing the existing bridge on NW Stringtown Road over Prickett Creek. The existing structure is approximately 17.5 feet wide by 31 feet long. We understand the bridge will be replaced with a three-sided box culvert that will have dimensions of 25 feet wide by 32 feet long and the culvert will be supported on spread footings.

2.0 PURPOSE AND SCOPE

The purpose of our geotechnical engineering services was to explore the subsurface conditions near the proposed improvements and develop geotechnical engineering recommendations for design and construction of the proposed culvert. Our scope included the following:

Coordinated and managed the field exploration, including locating utilities, access preparation, and scheduling subcontractors and NV5 field staff.

Prepared traffic control plans and obtained right-of-way permits from Washington County

(County).

Drilled one boring to a depth of 105 feet BGS for use in design of the structure.

Classified the material encountered in the exploration, maintained a detailed log of the exploration, and collected samples at representative intervals.

Conducted the following laboratory tests on soil samples collected from the exploration:

Fifteen moisture content determinations in general accordance with ASTM D2216 One grain-size analysis in general accordance with ASTM D422 Two Atterberg limits tests in general accordance with ASTM D4318

Provided geotechnical engineering construction recommendations for site preparation, structural fill compaction criteria, wet/dry weather earthwork procedures, cut and fill slopes, and suitability of on-site soil for use as structural fill.

Provided recommendations for design and construction of culvert foundations, including nominal bearing resistance and estimated settlement. Provided lateral earth pressures for headwall and culvert design.

Prepared this geotechnical engineering report that presents our findings, conclusions, and recommendations.

3.0 SITE CONDITIONS

3.1 GEOLOGIC SETTING

3.1.1 General

The site is located on the eastern flank of the Coast Range, adjacent to the Tualatin Basin located immediately east of the site. The site is bound by the Coast Range to the west, the

2 WashCo-97-01:120423

Chehalem Mountains to the south, and the Tualatin Basin to the east. The Gales Creek fault zone immediately east of site is generally interpreted as the boundary between the Coast Range and the Tualatin Basin (Wells et al., 2020b).

Bedrock beneath the site is mapped as the Yamhill Formation, a marine siltstone with interbeds of sandstone and sub-aerial basalt lava flows (Wells et al., 2020b). The Yamhill Formation was deposited offshore of the North American Continent approximately 20 million years ago, then subjected to accretion and uplift along the continental margin as a result of sustained subduction of the ancestral Farallon Plate. Uplift and seismic events continue to present day due to subduction by the antecedent Juan de Fuca Plate. Based on our subsurface exploration, the Yamhill Formation is locally overlain by clay colluvium resulting from the erosion of the Coast Range immediately west of the site. Weathered claystone of the Yamhill Formation is encountered at a depth of approximately 38 feet BGS in boring B-1, presented in the Appendix.

3.1.2 Faults and Seismicity

The northern Willamette Valley is seismically active due compressional tectonics caused by the subduction of the Juan de Fuca oceanic plate under the North American continental margin. The Tualatin Basin and surrounding areas have several northwest-striking faults identified below sedimentary cover and mapped in exposed bedrock (Yeats et al., 1991). The closest of these faults to the site is the Gales Creek fault complex, located less than a 0.25 mile northeast of the site. The Gales Creek fault zone is part of a larger series of northwest- to southeast-trending faults that bisect the Tualatin and Portland Basins east of the site. These faults are interpreted as re-activated normal faults accommodating compression and clockwise rotation, resulting in the localized uplift of the nearby Tualatin Mountains and Portland Hills (Yeats et al., 1991, Wells et al., 2020b). This fault appears to be along a larger structural zone that extends southeastward to the Mount Angel fault (Wells et al., 2020a). In 1993, a magnitude 5.3 earthquake occurred near the town of Scotts Mills, approximately 40 miles southeast of the site.

This earthquake is attributed to displacement along the Mount Angel fault (Blakely et al., 2000).

Recent trenching and evaluation of the Gales Creek fault in the vicinity of the Scoggins Dam and Hag Lake reservoir indicate observed displacement of the fault within the last 10,000 years (Wells et al., 2020b).

3.2 SURFACE CONDITIONS

The area surrounding Stringtown Road bridge over Prickett Creek consists of agricultural land.

The bridge site is bordered to the south and west by residential properties with structures and to the north and east by cultivated agricultural fields. The Prickett Creek channel is presently approximately 18 feet wide and contains relatively slowly flowing water. The roadway elevation at the bridge alignment is approximately 235 feet and is relatively level.

3.3 SUBSURFACE CONDITIONS

3.3.1 General

Subsurface conditions were explored by drilling one boring (B-1) to a depth of 105 feet BGS approximately 20 feet west of the west abutment of the existing bridge as shown on Figure 2.

Laboratory testing to determine physical and engineering properties of the soil at the site was completed on select samples from the exploration. The exploration log and laboratory testing results are presented in the Appendix.

3 WashCo-97-01:120423

The general subsurface profile consists of 3 inches of AC and 6 inches of aggregate base below the roadway surface. Clay fill was encountered below the pavement section to a depth of

4.5 feet BGS. The fill is underlain by alluvial clay extending to a depth of 38 feet BGS, and decomposed claystone consisting of very stiff high plasticity silt was encountered below the alluvium and extends to a depth of 95 feet BGS. Claystone from the Yamhill Formation was encountered below the silt (decomposed claystone) and extends to the depth explored of 105 feet BGS.

3.3.2 Pavement Section

A pavement section was observed in the exploration consisting of 3 inches of AC overlying 6 inches of aggregate base.

3.3.3 Clay (Fill)

Fill consisting of stiff clay was encountered immediately below the pavement section. Laboratory testing of a select sample indicates the clay fill had an in-place moisture content of 33 percent at the time of our exploration.

3.3.4 Clay (Alluvium)

Clay alluvium was encountered below the recent fill to a depth 38 feet BGS. The clay is generally soft to medium stiff. Laboratory testing of select samples indicates the clay had in-place moisture contents between 36 and 68 percent at the time of our exploration. An Atterberg limits test on a select sample indicates the clay has a liquid limit of 59, a plastic limit of 27, and a plasticity index of 32.

3.3.5 Silt (Decomposed Claystone)

Decomposed claystone deposits of silt were encountered below the alluvium deposits to a depth of 95 feet BGS. The silt is generally very stiff to hard. Laboratory testing of select samples indicates the silt had in-place moisture contents between 32 and 39 percent at the time of our exploration. The silt has high plasticity based on Atterberg limits test results with a liquid limit of 76, a plastic limit of 36, and a plasticity index of 40.

3.3.6 Claystone (Yamhill Formation)

Extremely soft to medium hard claystone of the Yamhill Formation was encountered below the silt to the maximum depth explored of 105 feet BGS. The claystone is intensely weathered to decomposed and moderately to very intensely fractured.

3.3.7 Groundwater

The presence of groundwater was difficult to directly determine due to the mud rotary drilling techniques in the exploration. Based on moisture increases in the collected samples, we believe the groundwater ranges from approximately 5 to 10 feet BGS. We anticipate that groundwater levels at the site will likely be consistent with the water level in Prickett Creek and could fluctuate during the wet and dry seasons.

4 WashCo-97-01:120423

4.0 CONCLUSIONS

Based on the results of our exploration and understanding of the project, the following factors will have an impact on design and construction of the proposed bridge replacement.

The key geotechnical consideration for the proposed culvert will be settlement of the highly compressible native clay soil from proposed loading from headwalls and culvert foundations.

We recommend that the native medium stiff clay be removed to a depth of 2 feet below the culvert and headwall foundations and replaced with granular backfill or stone embankment material.

Due to the presence of soft and wet soil, flatter than normal temporary cuts may be required, resulting in larger backfill quantities. We recommend that a contingency be in place for larger than anticipated backfill quantities. As an alternative, temporary shoring can be used to reduce temporary excavation quantities.

The existing soil at the site is fine grained and easily disturbed by construction traffic. If not carefully executed, site preparation, grading, and culvert roadway excavation in this soil can create extensive soft areas and significant subgrade repair costs can result.

Our design and construction recommendations for the project are provided in the following sections.

5.0 DESIGN

5.1 PERMANENT SLOPES

Permanent fill slopes on the site should not exceed a gradient of 2H:1V, unless specifically evaluated for stability. Slopes should be planted with appropriate vegetation or armored to provide protection against erosion as soon as possible after grading. Surface water runoff should be collected and directed away from slopes to prevent water from running down the face of the slope.

5.2 SEISMIC CONSIDERATIONS

5.2.1 Geologic Hazards

5.2.1.1 General

In addition to ground shaking, site-specific geologic conditions can influence the potential for earthquake damage. Deep deposits of loose or soft alluvium can amplify ground motions, resulting in increased seismic loads on structures. Other geologic hazards are related to soil failure and permanent ground deformation. Permanent ground deformation could result from liquefaction, lateral spreading, landsliding, and fault rupture. The following sections provide discussion regarding potential seismic hazards.

5.2.1.2 Fault Surface Rupture

Fault surface rupture is a seismic hazard that includes abrupt, large, differential ground movements that can cause damage to structures located on a known fault. Based on Wells et al.

(2020b), the closest mapped fault is the Gales Creek fault, which is located approximately

0.15 mile north of the site. Due to the distance of the fault, the risk of fault rupture is moderate to high at the site.

5 WashCo-97-01:120423

5.2.1.3 Liquefaction and Lateral Spreading

Liquefaction can be defined as the sudden loss of shear strength in a soil due to an excessive buildup of pore water pressure. Liquefied soil layers generally follow a path of least resistance to dissipate pore pressures, often resulting in sudden surface settlement, sand boils or ejections, and/or lateral spreading in extreme cases. Clean, loose, uniform or silty, fine-grained, saturated sand is particularly susceptible to liquefaction.

Based on the anticipated depth to groundwater and the subsurface conditions observed below the groundwater level, it is our opinion that the potential for liquefaction at the site is low. As the potential for liquefaction is low, the potential for lateral spreading is low and not considered a hazard for the site.

5.2.2 Seismic Design Parameters

Earthquake ground motions at the site were evaluated using seismic data from the 2016 ODOT Seismic Design maps. Peak bedrock accelerations for return periods of 500 years and 1,000 years for the “Life Safety” criteria are provided in Table 1. Based on geologic mapping and our exploration, the recommended seismic design Site Class is D. The “Life Safety” seismic design parameters for Site Class D soil are provided in Table 2.

Table 1. ODOT 2016 Seismic Design Parameters for Bedrock “Life Safety” Criteria

Parameter Value

Site Class (Bedrock) B

Return Period 500 years 1,000 years

PGA 0.187 g 0.302 g

Ss 0.402 g 0.654 g

S1 0.146 g 0.255 g

6 WashCo-97-01:120423

Table 2. ODOT 2016 Seismic Design Parameters for Soil “Life Safety” Criteria

Parameter Value

Site Class (Soil) D

Return Period 500 years 1,000 years

FPGA 1.426 1.298

Fa 1.478 1.277

Fv 2.308 2.091

SDS 0.594 g 0.835 g

SD1 0.337 g 0.532 g

As 0.267 g 0.392 g

5.3 CULVERT FOUNDATION RECOMMENDATIONS

5.3.1 Bearing Resistance

Based on the project plans, the bottom of footings will be established approximately 12.5 feet below the finished ground surface. We recommend that a minimum 2-foot-thick layer of crushed rock or stone embankment be placed below all footings for construction purposes. Footings should be designed to exert a nominal bearing resistance of 6,000 psf. A resistance factor of

0.5 should be applied to the nominal bearing resistance to calculate factored bearing resistance.

In order to prevent migration of fines, we recommend that a geotextile separation fabric be placed between the native fine-grained soil and crushed rock. Lateral loads on footings can be resisted by passive earth pressure on the sides of the structures and by friction on the base of the footings. Our analysis indicates that the available passive earth pressure for footings confined by on-site soil below the water table is 115 pcf, modeled as an equivalent fluid pressure. For footings in contact with crushed rock pads, a coefficient of friction equal to 0.40 may be used when calculating resistance to sliding.

5.3.2 Culvert Settlement

Based on the subsurface conditions and assuming the culvert foundations are underlain by 2 feet of granular material, settlement of up to 2 inches could occur as a result of the culvert and headwall loading. Differential settlement is estimated at one-half of the total settlement, and up to approximately 1 inch of differential ground settlement may occur. Our analysis indicates that the total settlement of the approach fills constructed on granular backfill overlying native clay will be approximately 2 inches for a nominal bearing resistance of 6,000 psf.

5.4 RETAINING STRUCTURES

5.4.1 General

Based on information provided by Otak, we understand that precast walls are planned to support the fills for the culvert.

7 WashCo-97-01:120423

5.4.1.1 Precast Wall Design Parameters

Our precast retaining wall design recommendations are based on the following assumptions:

(1) the walls consist of precast concrete walls, (2) the walls will be less than 8 feet in height, and

(3) the appropriate wall surcharges are included in the design as described in this section.

Precast walls should be designed using the soil parameters provided in Table 3. The material used to backfill behind all walls is discussed in the “Culvert and Headwall Select Backfill” section.

Table 3. Precast Wall Design Parameters

Soil Type Unit Weight

(pcf)

Friction Angle, ϕ (degrees)

Cohesion (psf)

Retained soil 125 34 0 Foundation soil

(granular fill overlying native clay)1,2 125 30 0

1. Wall backfill should meet the specifications provided in OSSC 00510.12 (Granular Wall Backfill).

2. Wall foundation subgrade should consist of 2 feet of granular backfill subgrade stabilization prepared in conformance with the “Site Preparation” section.

5.4.1.2 Wall Surcharges

Where traffic loads are located within a horizontal distance from the top of the wall equal to one-half the wall height (not anticipated), the lateral earth pressure shall be increased by a surcharge load equal to 2 feet of soil (assuming a soil density of 125 pcf).

Seismic forces should be modeled based on the pseudo-static approach developed by the Mononobe-Okobe method. We recommend using a seismic coefficient of one-half of the PGA when analyzing internal stability. The PGA should be modified to account for potential soil amplification using the seismic site coefficients described in “Seismic Considerations” section.

5.4.1.3 Temporary Cuts

Temporary cuts may be required to construct the proposed retaining walls. Excavations into the slopes need to be carefully planned so as not to destabilize the slopes. Cuts less than 4 feet should stand vertical. Deeper excavations should initially be cut back at an inclination 1.5H:1V or flatter or be shored. Depending on the depth of cuts and the soil encountered, excavation inclinations may be able to be steepened to 1.25H:1V or even 1H:1V. NV5 should be contacted at the time of excavation to observe the soil and determine if the steeper slope excavations are feasible. Because of the adverse effects that water can have on slope stability, we anticipate that this process will not be feasible for wintertime construction and wall construction should occur in dry summer months or temporary shoring used.

8 WashCo-97-01:120423

5.4.1.4 Construction Considerations

All footing subgrades should be evaluated by qualified personnel to confirm suitable bearing conditions. Observations should also confirm that all loose or soft material, organic material, unsuitable fill, prior topsoil zones, and softened subgrades (if present) have been removed.

Localized deepening of footing excavations may be required to penetrate deleterious material.

If construction is undertaken during periods of rain, we recommend placing at least 3 to 4 inches of imported granular material over prepared footing subgrades to help protect the subgrade from disturbance due to the elements and foot traffic. The imported material should meet the specifications for imported granular material as discussed in the “Materials” section.

5.5 DRAINAGE

During grading, the contractor should be responsible for temporary drainage of surface water as necessary to prevent standing water and/or erosion at the working surface. Surface runoff should not be allowed to flow over the face of the exposed soil.

5.6 PAVEMENT DESIGN RECOMMENDATIONS

5.6.1 General

Our recommendations for design and construction of new pavement for culvert approaches are presented in this section. These recommendations should be incorporated into design and implemented during construction of the proposed improvements. The recommended flexible pavement below should conform to the specifications presented in the “Materials” section.

5.6.2 Design Standards

The standards used for pavement design are listed below:

AASHTO guide (AASHTO, 1993) ODOT guide (ODOT, 2019) Washington County Road Design and Construction Standards (2011), where applicable

5.6.3 Design Life and ESAL Calculations

We received traffic count data from the prime consultant (Otak). We used vehicle classification data from ODOT’s TransGIS website. Table 4 summarizes the traffic data used in our ESAL calculations. ESALs were calculated for a design life of 20 years using the ODOT method. Table 5 provides the design ESALs.

Table 4. Traffic Input Summary

Traffic Data Collection Location 2017 Two-Way

AADT

2021 Two-Way

AADT

Truck

Percentage NW Stringtown Road 358 376 10.6

Table 5. Design ESALs

Traffic Data Collection Location 20-Year Design ESAL

NW Stringtown Road 76,000

9 WashCo-97-01:120423

5.6.4 Subgrade Resilient Modulus

We observed that the native subgrade soil (up to 10 feet BGS) primarily consists of medium stiff mottled clay with trace amounts of fine sand. We estimated the subgrade resilient modulus to be 4,000 psi.

5.6.5 Other Design Parameters

Other pavement design parameters used in our analysis are summarized in Table 6. These input parameters are recommended in the ODOT guide.

Table 6. Other Design Parameters

Parameter Design Value

Design reliability level, percent 85 (Rural Major Collector) Initial serviceability, Po 4.2

Terminal serviceability, Pt 2.5 Standard deviation 0.49

New AC layer coefficient 0.42 New aggregate base layer coefficient 0.10

New aggregate base resilient modulus, psi 20,000

5.6.6 Pavement Design

We used the AASHTO method of pavement design along with the data mentioned above to determine the new flexible pavement for this project. The following design is for new 20-year pavement with minimum AC thickness:

4.0 inches of AC

12.0 inches of aggregate base Subgrade stabilization (if necessary) Subgrade geotextile

6.0 CONSTRUCTION

6.1 EROSION CONTROL

When exposed, the soil at this site can easily be eroded by wind and water; therefore, erosion control measures should be carefully planned and in place before construction begins. Where feasible, existing pavement and aggregate base should be left in place to protect the ground surface. Measures employed to reduce erosion include, but are not limited to, silt fences, hay bales, plastic sheeting, buffer zones of natural growth, and sedimentation ponds.

6.2 SITE PREPARATION

6.2.1 Demolition

The limits of the required demolition should be determined by the project engineer, although they should include all improvements that will impede construction of the improvements. Within structural areas (new pavement, foundations, and fills), excavations resulting from removing existing bridge and other subsurface elements should be backfilled with compacted engineered

10 WashCo-97-01:120423 fill that meets the criteria outlined in the “Materials” section and as described in OSSC 00310 (Removal of Structures and Obstructions). In addition, the bottoms of excavations should expose firm subgrade before filling. The sides of excavations should be cut into firm material and sloped at a minimum gradient of 1H:1V to allow for more uniform compaction of backfill. Utility lines abandoned under new structural components should be completely removed or (with written approval) grouted full if left in place.

6.2.2 Clearing

Stripping and clearing shall be completed in conformance with the specifications provided in OSSC 00320 (Clearing and Grubbing). Limited areas of exposed soil are currently present in the proposed work area. We anticipate that topsoil stripping depths will be 4 to 6 inches. However, greater stripping depths may be encountered. The actual stripping depth should be based on field observations at the time of construction. The existing pavement section should be removed prior to placement of AC and aggregate base.

Stripped material should be transported off site for disposal or placed as topsoil dressing as directed by the project engineer and the County.

6.2.3 Subgrade Evaluation

Upon completion of stripping and prior to the placement of foundation, fill, or pavement, the exposed subgrade should be evaluated by proof rolling or probing with a foundation probe to identify soft, loose, or unsuitable areas. A proof roll should be completed with a fully loaded dump truck or similar heavy, rubber tire construction equipment. The proof roll should be completed in conformance with the specifications provided in OSSC 00330.43 (Earthwork Compaction Requirements). Qualified personnel should observe proof rolling to evaluate yielding of the ground surface.

If soft or yielding subgrade areas are identified during evaluation, we recommend the subgrade be excavated and replaced with compacted imported granular fill in conformance with stabilization material as described in the “Materials” section and the specifications provided in OSSC 00331 (Subgrade Stabilization). Scarification and compaction of soft subgrade should not be attempted, unless the material can be aerated and dried during extended periods of dry weather.

6.3 WET WEATHER CONSIDERATIONS

The fine-grained soil at the site is easily disturbed during the wet season or when the moisture content of the material is more than a few percentage points above optimum. This will likely be throughout the year, except between mid-summer through early fall (typically from early July to mid-October). If not carefully executed, site preparation and construction work can create extensive soft areas and significant subgrade repair costs can result. If construction is planned when the surficial soil is wet or may become wet, the construction methods and schedule should be carefully considered with respect to protecting the subgrade to reduce the need to over-excavate disturbed or softened soil. The project budget should reflect the recommendations below if construction is planned during wet weather or when the surficial soil is wet.

11 WashCo-97-01:120423

In general, a 12- to 18-inch-thick granular pad is sufficient for light staging areas but is not expected to be adequate to support heavy equipment or truck traffic for haul roads and areas with repeated heavy construction. In our experience, an 18- to 24-inch section should be adequate. The actual thickness of haul roads and staging areas should be based on the contractor’s approach to site development and the amount and type of construction traffic.

Consequently, the contractor should be responsible for selecting the locations of staging areas and haul roads and the appropriate thickness of granular material for these areas.

The imported granular material should be placed in one lift over the prepared, undisturbed subgrade and compacted using a smooth-drum roller without the use of vibratory action. The granular material should meet the specifications for imported granular material in the “Materials” section. In addition, a geotextile fabric can be placed as a barrier between the subgrade and granular material in areas of repeated construction traffic, if required. The geotextile should have a minimum Mullen burst strength of 250 psi and an AOS between U.S. Standard No. 70 and No. 100 sieves.

6.4 EXCAVATION

6.4.1 General Excavation

Based on our exploration, the subsurface soil at the site consists of fine-grained soil. Excavation in the site soil should be possible with conventional earthwork equipment. However, we observed sensitive soil during our exploration, and sloughing, caving, or “running” of excavation sidewalls should be expected for cuts deeper than approximately 10 feet BGS and especially if perched groundwater is encountered.

Trench cuts and footing excavations should stand vertical to a depth of approximately 4 feet, provided groundwater seepage does not occur in the trench walls. Open excavations may be used to excavate trenches with depths between 4 and 8 feet, provided the walls of the excavation are cut at a slope of 1.5H:1V, groundwater seepage does not occur, and with the understanding that some sloughing may occur.

If sloughing or caving occurs, we anticipate that temporary shoring will be used during excavation. If shoring is used, we recommend that the type and design of the shoring system be the responsibility of the contractor, who is in the best position to choose a system that fits the plan of operation.

6.4.2 Dewatering

If groundwater or perched water is encountered, dewatering of excavations may be required.

Sidewalls of the trenches will need to be shored or flattened if seepage is encountered.

Sloughing and “running” conditions could occur where the excavation extends below groundwater seepage levels because of the instability of saturated, loose sand present beneath the ground surface. If these conditions exist, shoring and an active dewatering system will likely be required for deep trenches. Pumping from a sump located within the trench will likely not be effective at controlling sloughing and “running” conditions, and a well point may be required.

If groundwater is encountered at the base of excavations, we recommend over-excavating the subgrade by 12 to 18 inches and placing stabilization rock in the base.

12 WashCo-97-01:120423

6.4.3 Safety

All excavations should be made in accordance with applicable OSHA and state regulations.

While we have described certain approaches to excavations in the foregoing discussions, the contractor is responsible for selecting the excavation and dewatering methods, monitoring the trench excavations for safety, and providing shoring as required to protect personnel and adjacent structures.

6.5 MATERIALS

6.5.1 Structural Fill

A variety of material may be used as structural fill at the site. Fill should only be placed over subgrade that has been prepared in conformance with the “Site Preparation” section. Structural fill should meet the specifications provided in the OSSC 00330 (Earthwork), OSSC 00400 (Drainage and Sewers), and OSSC 02600 (Aggregates), depending on the application. A brief characterization of some of the acceptable materials and our recommendations for their use as structural fill are provided below.

6.5.1.1 On-Site Soil

The material at the site should be suitable for use as general embankment fill, provided it is properly moisture conditioned, free of organic material and particles over 6 inches in diameter, and meets the specifications provided in OSSC 00330.12 (Borrow Material). Laboratory testing indicates the moisture content of the on-site soil is higher than the optimum moisture required for compaction as structural fill. Therefore, moderate to extensive moisture conditioning (i.e., drying) of the on-site soil will be required in order to achieve proper compaction; therefore, the use of on-site soil for structural fill will be restricted to periods of prolonged dry weather.

When used as structural fill, native soil should be placed in lifts with a maximum uncompacted thickness of 6 to 8 inches and compacted to not less than 95 percent of the maximum dry density, as determined by AASHTO T 99.

6.5.1.2 Imported Granular Material

Imported granular material used as structural fill should be pit- or quarry-run rock, crushed rock, or crushed gravel and sand and should meet the specifications provided in OSSC 00330.14 (Selected Granular Backfill) or OSSC 00330.15 (Selected Stone Backfill). The imported granular material should also be angular, should be fairly well graded between coarse and fine material, should have less than 5 percent by dry weight passing the U.S. Standard No. 200 sieve, and should have at least two mechanically fractured faces.

Imported granular material should be placed in lifts with a maximum uncompacted thickness of 8 inches and compacted to not less than 95 percent of the maximum dry density, as determined by AASHTO T 99. During the wet season or when wet subgrade conditions exist, the initial lift should be approximately 12 to 18 inches in uncompacted thickness and should be compacted by rolling with a smooth-drum roller without using vibratory action.

Where the imported granular material is placed atop a fine-grained subgrade, a geotextile should be placed as a barrier between the native soil subgrade and the imported granular material.

Placement of the imported granular fill should be done in conformance with the specifications

13 WashCo-97-01:120423 provided in OSSC 00331 (Subgrade Stabilization). The geotextile should meet the specifications provided in OSSC 02320.20 (Geotextile Property Values) for soil separation. The geotextile should be installed in conformance with OSSC 00350 (Geosynthetic Installation).

6.5.1.3 Stabilization Material

Stabilization material should consist of pit- or quarry-run rock, crushed rock, or crushed gravel and should meet the specifications provided in OSSC 00330.16 (Stone Embankment Material).

In addition, the material should have a maximum particle size of 6 inches, should have less than 5 percent by dry weight passing the U.S. Standard No. 4 sieve, and should have at least two mechanically fractured faces. The material should be free of organic material and other deleterious material. Stabilization material should be placed in lifts between 12 and 18 inches thick and compacted to a firm condition.

Where stabilization material is used to stabilize soft subgrade beneath pavement or construction haul roads, a geotextile should be placed as a barrier between the soil subgrade and the imported granular material. Placement of the imported granular fill should be done in conformance with the specifications provided in OSSC 00331 (Subgrade Stabilization). The geotextile should meet the specifications provided in OSSC 02320.20 (Geotextile Property Values for soil separation). The geotextile should be installed in conformance with OSSC 00350 (Geosynthetic Installation). Geotextile is not required where stabilization material is used at the base of utility trenches.

6.5.1.4 Trench Backfill

Trench backfill placed beneath, adjacent to, and for at least 12 inches above utility lines (i.e., the pipe zone) should consist of well-graded granular material with a maximum particle size of 1 inch and less than 10 percent by dry weight passing the U.S. Standard No. 200 sieve and should meet the specifications provided in OSSC 00405.13 (Pipe Zone Material). The pipe zone backfill should be compacted to at least 90 percent of the maximum dry density, as determined by AASHTO T 99, or as required by the pipe manufacturer or local agency.

Within roadway alignments, the remainder of the trench backfill up to the subgrade elevation should consist of well-graded granular material with a maximum particle size of 2½ inches and less than 7 percent by dry weight passing the U.S. Standard No. 200 sieve and should meet OSSC 00405.14 (Trench Backfill, Class B, C, or D). This material should be compacted to at least 90 percent of the maximum dry density, as determined by AASHTO T 99, or as required by the pipe manufacturer or local agency. The upper 3 feet of the trench backfill should be compacted to at least 95 percent of the maximum dry density, as determined by AASHTO T 99.

6.5.1.5 Culvert and Headwall Select Backfill

Backfill material placed behind culverts and headwalls and extending a horizontal distance of ½H, where H is the height of the retaining wall, should consist of imported granular material as described above and should have less than 7 percent fines by dry weight. We recommend the wall backfill be separated from general fill, native soil, and/or topsoil using a geotextile fabric that meets the specifications provided below for drainage geotextiles.

14 WashCo-97-01:120423

Wall backfill should be compacted to a minimum of 95 percent of the maximum dry density, as determined by AASHTO T 99. However, backfill located within a horizontal distance of 3 feet from a retaining wall should only be compacted to approximately 90 percent of the maximum dry density, as determined by AASHTO T 99. Backfill placed within 3 feet of the wall should be compacted in lifts less than 6 inches thick using hand-operated tamping equipment (such as a jumping jack or vibratory plate compactor). If flatwork (sidewalks or pavement) will be placed atop the wall backfill, we recommend the upper 2 feet of material be compacted to 95 percent of the maximum dry density, as determined by AASHTO T 99.

6.5.1.6 Drain Rock Material

Drain rock should consist of angular, granular material with a maximum particle size of 2 inches.

The material should be free of roots, organic material, and other unsuitable material; should have less than 2 percent by dry weight passing the U.S. Standard No. 200 sieve (washed analysis); and should have at least two mechanically fractured faces. Drain rock should be compacted to a well-keyed, firm condition.

6.5.2 Geotextile Fabric

6.5.2.1 Drainage Geotextile

Drainage geotextile should conform to Type 2 material of OSSC Table 02320-1 and OSSC 00350 (Geosynthetic Installation). A minimum initial aggregate base lift of 6 inches is required over geotextiles.

6.5.3 AC

6.5.3.1 ACP

The AC should be Level 3, ½-inch, dense ACP according to OSSC 00744 (Asphalt Concrete Pavement). The AC should be compacted to 92 percent of the theoretical maximum density of the mix, as determined by AASHTO T 209. The minimum and maximum lift thicknesses are 2 and 3.5 inches, respectively, for ½-inch ACP. Asphalt binder should be performance graded and conform to PG 64-22. The binder grade should be adjusted depending on the aggregate gradation and amount of recycled asphalt pavement and/or recycled asphalt shingles in the contractor’s mix design submittal.

6.5.3.2 Cold Weather Paving Considerations

In general, AC paving is not recommended during cold weather (temperatures less than 40 degrees Fahrenheit). Compacting under these conditions can result in low compaction and premature pavement distress.

Each AC mix design has a recommended compaction temperature range that is specific for the particular AC binder used. In colder temperatures, it is more difficult to maintain the temperature of the AC mix as it can lose heat while stored in the delivery truck, as it is placed, and in the time between placement and compaction. In Oregon, the AC surface temperature during paving should be at least 40 degrees Fahrenheit for lift thickness greater than 2.5 inches and at least 50 degrees Fahrenheit for lift thickness between 2 and 2.5 inches.

15 WashCo-97-01:120423

If paving activities must take place during cold weather construction as defined above, the project team should be consulted and a site meeting should be held to discuss ways to lessen low compaction risks.

7.0 OBSERVATION OF CONSTRUCTION

Satisfactory earthwork and pavement performance depends to a large degree on the quality of construction. Sufficient observation of the contractor's activities is a key part of determining that the work is completed in accordance with the construction drawings and specifications.

Subsurface conditions observed during construction should be compared with those encountered during the subsurface explorations. Recognition of changed conditions often requires experience; therefore, qualified personnel should visit the site with sufficient frequency to determine if subsurface conditions change significantly from those anticipated.

We recommend that NV5 be retained to observe earthwork activities, including stripping, proof rolling of the subgrade and repair of soft areas, final proof rolling of the pavement subgrade and aggregate base, and retaining wall footing excavations, and performing laboratory compaction and field moisture-density testing.

8.0 LIMITATIONS

We have prepared this report for use by Washington County and the design and construction team for the proposed project. The report can be used for bidding or estimating purposes, but our report, conclusions, and interpretations should not be construed as warranty of the subsurface conditions and are not applicable to other sites.

Exploration observations indicate soil conditions only at specific locations and only to the depths penetrated. They do not necessarily reflect soil strata, pavement, or water level variations that may exist between exploration locations. If subsurface conditions differing from those described are noted during the course of excavation and construction, re-evaluation will be necessary.

The improvement plans and design details were preliminary at the time this report was prepared.

When the design has been finalized and if there are changes in the site grades, location, or configuration, the conclusions and recommendations presented may not be applicable. If design changes are made, we request that we be retained to review our conclusions and recommendations and to provide a written modification or verification.

The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractor's methods, techniques, sequences, or procedures, except as specifically described in this report for consideration in design.

Within the limitations of scope, schedule, and budget, our services have been executed in accordance with generally accepted practices in this area at the time this report was prepared.

No warranty, express or implied, should be understood.

16 WashCo-97-01:120423

We appreciate the opportunity to be of continued service to you. Please call if you have questions concerning this report or if we can provide additional services.

Sincerely, NV5

Jeffery D. Tucker, P.E., G.E.

Principal Engineer

17 WashCo-97-01:120423

REFERENCES

AASHTO, 1993. AASHTO Guide for Design of Pavement Structures.

ODOT, 2016. 2016 ODOT Seismic Design Maps, 500 Year Peak Ground Acceleration, Oregon Department of Transportation, 2016.

ODOT, 2016. 2016 ODOT Seismic Design Maps, 500 Year 0.2 SEC. Spectral Acceleration, Oregon Department of Transportation, 2016.

ODOT, 2016. 2016 ODOT Seismic Design Maps, 500 Year 1.0 SEC. Spectral Acceleration, Oregon Department of Transportation, 2016.

ODOT, 2016. 2016 ODOT Seismic Design Maps, 1,000 Year Peak Ground Acceleration, Oregon Department of Transportation, 2016.

ODOT, 2016. 2016 ODOT Seismic Design Maps, 1,000 Year 0.2 SEC. Spectral Acceleration, Oregon Department of Transportation, 2016.

ODOT, 2016. 2016 ODOT Seismic Design Maps, 1,000 Year 1.0 SEC. Spectral Acceleration, Oregon Department of Transportation, 2016.

ODOT, 2019. ODOT Pavement Design Guide, Oregon Department of Transportation, January 2019.

ODOT, 2021. Oregon Standard Specifications for Construction, Oregon Department of Transportation, 2021 Edition.

Road Design and Construction Standards, Washington County Department of Land Use and Transportation Public Works Department, February 2011.

Wells, R.E., Blakely, R.J., and Bemis, S., 2020a, Northward migration of the Oregon forearc on the Gales Creek fault. Geosphere (2020); 16 (2): 660–684.

Wells, R.E., Haugerund, R.A., Niem, A.R., Niem, W.A., Ma, L., Evarts, R.C., O’Connor, J.E., Madin, I.P., Sherrod, D.R., Beeson, M.H., Tolan, T.L., Wheeler, K.L., Hanson, W.B., and Sawlan, M.G., 2020b, Geologic Map of the Greater Portland Metropolitan Area and Surrounding Region, Oregon and Washington. U.S. Geologic Survey, Scientific Investigations Map 3443, 3 Plates, Scale 1:63,360.

Yeats, R.S., Graven, E.P., Warner, K.S., Goldfinger, C., and Popowski, T., 1991, Tectonics of the Willamette Valley, Oregon. U.S. Geologic Survey, Open File Report OF-91-441-P, 8 plates, Scale 1:100,000.

FIGURES

VICINITY MAP BASED ON AERIAL

PHOTOGRAPH OBTAINED FROM

GOOGLE EARTH PRO®

SITE

Pr in te d B y:

a n d y.

d ay

P ri n t

D at e:

/4

/2

:4

:3

P

M

Fi le

N am e:

J:

\S

-Z \W as h C o \W as h C o -9

\W as h C o -9

-0

\F ig u re s\ C

A D

\W as h C o -9

-0

-V

M

.d w g L ay o u t:

F

IG

U R

E

VICINITY MAP

STRINGTOWN ROAD BRIDGE REPLACEMENT

WASHINGTON COUNTY, OR

WASHCO-97-01

DECEMBER 2023 FIGURE 1

(SCALE IN APPROXIMATE FEET)

N

2000 4000

B-1

N W

S

YL

VI

A

LA

N E

PR

IC

K

ET

T R

O A D

NW

S

T R

IN

G T O W

N

RO

A D

Pr in te d B y:

a n d y.

d ay ri n t

D at e:

/4

/2

:4

:3

P

M

Fi le

N am e:

J:

\S

-Z \W as h C o \W as h C o -9

\W as h C o -9

-0

\F ig u re s\ C

A D

\W as h C o -9

-0

-S

P0

.d w g L ay o u t:

F

IG

U R

E

S

IT

E P

L A

N

ST

R

IN

G

T O

W N

R O

A D

B R

ID

G

E R

EP

LA

C

EM

EN

T

W A

SH

IN

G T

O N

C O

U N

T Y

O R

W A

SH

C

O -9

-0

D

EC

EM

B

ER

F

IG

U R

E

(SCALE IN FEET)

N

100 200

SITE PLAN BASED ON AERIAL PHOTOGRAPH DATED

JUNE 17, 2021, OBTAINED FROM GOOGLE EARTH PRO.

LEGEND:

BORINGB-1

APPENDIX

A-1 WashCo-97-01:120423

APPENDIX

FIELD EXPLORATIONS

GENERAL

We explored subsurface conditions at the site by drilling one boring (B-1) to a depth of 105 feet BGS. The boring was completed by Western States Soil Conservation, Inc. of Hubbard, Oregon, using mud rotary and HQ core drilling techniques. The exploration log is presented in this appendix. The boring was completed under the supervision of NV5 personnel. We chose the locations of the exploration based on information provided by the design team.

SOIL AND ROCK SAMPLING

We collected representative samples of the various soils encountered during drilling in the exploration for geotechnical laboratory testing. Samples were collected from the boring using 1½-inch-inside diameter split-spoon SPT sampler in general accordance with ASTM D1586. The samplers were driven into the soil with a 140-pound automatic trip hammer free falling 30 inches. The sampler was driven a total distance of 18 inches. The number of blows required to drive the sampler the final 12 inches is recorded on the exploration log, unless otherwise noted. Higher quality, relatively undisturbed samples were collected using a standard Shelby tube in general accordance with ASTM D1587.

The average efficiency of the automatic SPT hammer used by Western States Soil Conservation, Inc. was 79.2 percent. The calibration testing results are presented at the end of this appendix.

Bedrock was cored continuously using HQ core drilling methods in general accordance with ASTM D2113. Percent core recovery and RQD are noted on the exploration log. The RQD is defined as the total length of all the intact core sections over 4 inches in length divided by the total length of the core run. The retrieved rock core was logged and stored in core boxes for later analysis.

Sampling methods and intervals are shown on the exploration log.

SOIL AND ROCK CLASSIFICATION

The soil and rock samples were classified in the field in accordance with the “Exploration Key” (Table A-1), “Soil Classification System” (Table A-2), and “Rock Classification System” (Table A-3), which are presented in this appendix. The exploration logs indicate the depths at which the soil characteristics change, although the change could be gradual. If the change occurred between sample locations, the depth was interpreted. Classifications are shown on the exploration logs.

LABORATORY TESTING

GENERAL

Laboratory testing was conducted on select soil samples to confirm field classifications and determine the index…